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Chapter 27 Prokaryotes - Chapter 27 Prokaryotes Lecture...

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Chapter 27 Prokaryotes Lecture Outline Overview: They’re (Almost) Everywhere! Prokaryotes were the earliest organisms on Earth. Today, they still dominate the biosphere. o Their collective biomass outweighs all eukaryotes combined at least tenfold. o More prokaryotes inhabit a handful of fertile soil or the mouth or skin of a human than the total number of people who have ever lived. Prokaryotes are wherever there is life. They thrive in habitats that are too cold, too hot, too salty, too acidic, or too alkaline for any eukaryote. o Prokaryotes have even been discovered in rocks two miles below the surface of the Earth. Why have these organisms dominated the biosphere since the origin of life on Earth? o Prokaryotes display diverse adaptations that allow them to inhabit many environments. o They have great genetic diversity. Prokaryotes are classified into two domains, Bacteria and Archaea, which differ in structure, physiology and biochemistry. Concept 27.1 Structural, functional, and genetic adaptations contribute to prokaryotic success Prokaryotes are small. Most prokaryotes are unicellular. o Some species may aggregate transiently or form true colonies, showing division of labor between specialized cell types. Most prokaryotes have diameters in the range of 1–5 ?m, compared to 10–100 ?m for most eukaryotic cells. o The largest prokaryote discovered so far has a diameter of 750 ?m. The most common shapes among prokaryotes are spheres (cocci), rods (bacilli), and helices. Nearly all prokaryotes have a cell wall external to the plasma membrane. In nearly all prokaryotes, a cell wall maintains the shape of the cell, affords physical protection, and prevents the cell from bursting in a hypotonic environment. In a hypertonic environment, most prokaryotes lose water and plasmolyze, like other walled cells. o Severe water loss inhibits the reproduction of prokaryotes, which explains why salt can be used to preserve foods.
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Most bacterial cell walls contain peptidoglycan, a polymer of modified sugars cross- linked by short polypeptides. o The walls of archaea lack peptidoglycan. The Gram stain is a valuable tool for identifying specific bacteria based on differences in their cell walls. o Gram-positive bacteria have simple cell walls with large amounts of peptidoglycans. o Gram-negative bacteria have more complex cell walls with less peptidoglycan. An outer membrane on the cell wall of gram-negative cells contains lipopolysaccharides, carbohydrates bonded to lipids. Among pathogenic bacteria, gram-negative species are generally more deadly than gram- positive species. o The lipopolysaccharides on the walls of gram-negative bacteria are often toxic, and the outer membrane protects the pathogens from the defenses of their hosts.
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